1. Field of the Invention
The present invention relates to a bicycle frame; more specifically, it pertains to a so-called diamond-type bicycle frame having a unique distribution of effective section modulus in the longitudinal direction of tube members, for realizing optimized rigidity and mechanical strength. The invention further relates to a method of, and an apparatus for manufacturing such bicycle frames.
2. Description of the Related Art
Typically, a diamond-type bicycle frame is composed of various tube members, including a head tube, a top tube, a down tube, a seat tube, a pair of seat stays and a pair of chain stays. The upper and lower ends of the head tube are integrally connected to the front end portions of the top tube and the down tube, respectively. A hanger lug is integrally connected to the rear end of the down tube, the lower end of the seat tube and the front ends of the respective chain stays. Each seat stay has a front end which is integrally connected to the rear end of the top tube or to the upper end of the seat tube, and also has a rear end which is integrally connected to the rear end of the relevant chain stay.
As widely known in the art, bicycle frames are generally classified into a lugged frame and a lugless frame. Thus, a diamond-type lugged frame incorporates head lugs between the head tube and the top tube and also between the head tube and the down tube, a seat lug between the top tube and the seat tube, and a hanger lug between the down tube and the seat tube. On the other hand, in the case of a diamond-type lugless frame, the head tube and the top tube, the head tube and the down tube, and the top tube and the seat tube are respectively butt-welded to each other, without interposing lugs between the relevant tubes. Known bicycle frames of such basic structures do not afford sufficient rigidity or mechanical strength required for severe use conditions (e.g., off-road driving as in a mountain bicycle), and it is difficult to improve the manufacturing productivity of such known frames. Furthermore, regardless of whether the bicycle frame is of a lugged-type or a lugless-type, there may arise a dangerous situation wherein adjacent tube members undergo separation from each other due at least partly to brazing or welding failures.
More particularly, a diamond-type lugged frame is often manufactured by using a head tube which is integrally provided with head lugs, wherein the diameter of each fitting bore of the seat lug and the hanger lug is so determined that a predetermined clearance is left between the lug and the tube member. Due to the presence of such a clearance, each tube member can be axially inserted into the fitting bore of the relevant lug by a gradually incremental amount, until a predetermined insertion depth is reached. By assembling a set of head tube, top tube, down tube and seat tube, there is formed a so-called front triangle. After formation and shaping of the front triangle, subsequent steps are carried out wherein seat stays and chain stays are secured to jigs as rear triangles and integrally connected to the front triangle by brazing, and the distortion of the entire frame is finally corrected.
During the formation of a lugged frame as above, each tube member assumes a fixed angular relationship with the relevant lug, particularly when the tube member has been deeply inserted into the lug. In a condition wherein the top tube and the down tube are rigidly retained by the seat lug and the hanger lug at both ends of the seat tube, respectively, since the head tube is integrally provided with head lugs, it is almost impossible to assemble the head tube in the final stage of the front triangle formation. It has therefore been a conventional practice to insert the front end portion of the top tube and the down tube into the respective head lugs and to simultaneously insert the rear end portion of the seat lug. Of course, the necessity for such simultaneous insertion of a plurality of tube members prohibits the realization of an automated assembly of the bicycle frames.
Apart from the above, it is widely known in the art that each tube member of a bicycle frame is subjected to various loads during use, of which a bending moment is a predominant load. Moreover, the bending stress occurring in the tube member assumes the maximum value at the axial end portions of the tube member and is relatively low in the intermediate portion thereof. Thus, in ordinary bicycle frames including a tube member with an effective section modulus which is substantially constant in its longitudinal direction, the tube member is designed to have a section modulus which is determined with reference to the maximum stress at the end portion. As a result, the intermediate portion of the tube member has an excessive strength which is in fact unnecessary, and which is one important factor prohibiting the realization of bicycle frames with minimized weight.